1. Field of the Invention
The present invention relates to a protective element wherein a heat-generating member generates heat under abnormal circumstances, which causes a low melting metal member to blow out.
2. Description of the Related Art
Protective elements comprising heat-generating members and low melting metal members layered on a substrate are known to be able to prevent overvoltages as well as overcurrents (e.g. Japanese Patent No. 2790433 and Japanese Patent Application Laid-open No. H8-161990). Such protective elements contain heat-generating members through which electricity is passed in abnormal circumstances, and the heat generated by these members melts the low melting metal. The surface of the electrode on which the low melting metal member is disposed is thereby wetted, which causes the low melting metal member to blow out.
FIG. 7 is a circuit diagram showing an example of an overvoltage-preventing device which utilises such a protective element 1p. FIGS. 8A and 8B show respectively a plane view and a sectional view of the protective element 1p. 
The protective element 1p has a structure comprising heat-generating members 3, on which a resist paste has been applied, an insulating layer 4, and a low melting metal member 5 comprising a fuse material, all of which are layered on a substrate 2. In the drawings, 6a and 6b are electrodes for the heat-generating members; the electrode 6b is connected to an electrode in the central portion of the low melting metal member 5 (intermediate electrode 7c), with the connection site being situated between two sites 5a and 5b into which the low melting metal member 5 has been divided. 7a, 7b and 7c are electrodes for the low melting metal member. 8 is a internal sealing component made from solid flux, with which the low melting metal member 5 is sealed to prevent its surface from oxidising, and 9 is an external sealing component comprising a material which has higher melting and softening points than the low melting metal member 5 and which prevents the molten low melting metal member 5 from flowing out of the protective element once it has blown out.
In the overvoltage preventing device shown in FIG. 7, which uses the protective element 1p, terminals A1 and A2 are connected to electrode terminals on the device to be protected; e.g. a lithium-ion battery, while terminals B1 and B2 are connected to electrode terminals of devices such as a charging device connected to the device to be protected. According to the overvoltage preventing device, when an overvoltage that is larger than the breakdown voltage is applied to a zener diode D once the charging of the lithium-ion battery has been started, an abrupt base current ib will flow and cause a large collector current ic to flow through the heat-generating members 3 and thereby cause the heat-generating members 3 to heat up. This heat is transmitted to the low melting metal member 5 on the heat-generating members 3, causing the two sites 5a and 5b of the low melting metal member 5 to blow out. This prevents any overvoltage from being applied to the terminals A1 and A2 and simultaneously interrupts the current flowing to the heat-generating members 3.
In order to stop the generation of heat by interrupting the flow of electricity destined for the heat-generating member at the same time that the low melting metal member blows out, such protective elements are constituted in such a way that an intermediate electrode divides the low melting metal member into two sites, both of which will blow out. An example of an embodiment of the connection between the low melting metal member and heat-generating members is taught in Japanese Patent Application Laid-open No. H10-116549 and Japanese Patent Application Laid-open No. H10-116550, wherein the low melting metal member and heat-generating members are both disposed planarly on a substrate, instead of having the low melting metal member being layered on top of the heat-generating members. However, the embodiment is otherwise the same insofar as the low melting metal member blows out in two locations, so as to interrupt the flow of electricity destined for the heat-generating members at the same time that the low melting metal member blows out.
Similarly to the protective element 1p shown in FIGS. 8A and 8B, FIGS. 9A and 9B depict respectively a plane view and a sectional view of a protective element 1q, in which the heat generated from a flow of electricity passing through a heat-generating member 3 causes a low melting metal member 5 to blow out and the flow of electricity destined for the heat-generating member 3 to be simultaneously interrupted (Japanese Patent Application No. H10-110163). Low melting metal member electrodes 7a, 7b and 7c are furnished on a substrate 2 in this protective element 1q, and a low melting metal member 5 (5a, 5b) is disposed so as to bridge these electrodes 7a, 7b and 7c. A heat-generating member 3 is furthermore furnished on the underside of the low melting metal member electrode (intermediate electrode) 7c, with an insulating layer 4 interposed therebetween. The heat-generating member 3 is heated by the flow of electricity passed between the heat-generating member electrode 6b and the leads 6x and 6y coming from the heat-generating member electrode 6a. The heat-generating member electrode 6b is connected to the low melting metal member electrode (intermediate electrode) 7c. Accordingly, the heat generated by the heat-generating member 3 causes both the low melting metal member 5a between the electrodes 7a and 7c, and the low melting metal member 5b between the electrodes 7b and 7c to blow out, and thereby interrupt the flow of electricity passed to the device to be protected, while also interrupting the flow of electricity transmitted to the heat-generating member 3.
However, in such conventional protective elements 1p and 1q as described in the foregoing, there are a total of two locations where the low melting metal member 5 are supposed to blow out when heat is generated by the heat-generating member 3: one location in the middle of the low melting metal member 5a which bridges the low melting metal member electrodes 7a and 7c and one location in the middle of the low melting metal member 5b which bridges the low melting metal member electrodes 7b and 7c; there is an equal number of locations which are able to blow out. Therefore, if a location which is supposed to blow out inexplicably becomes unable to blow out, and thus in abnormal circumstances fails to blow out, the device to be protected may be irretrievably damaged.
In view of the foregoing problem, it is an object of the present invention to ensure that the number of places on the low melting metal member that are able to blow out is greater than the number of places that are supposed to blow out, thereby imparting a so-called xe2x80x9cfail-safexe2x80x9d function to the protective element.
The inventors of the present invention discovered that by forming a portion of the low melting metal member on top of a dummy electrode, locations on the low melting metal member which are able to blow out will be formed on either side of the dummy electrode, and thereby the number of locations which are able to blow out will be greater than the number of locations which are supposed to blow out.
In other words, the present invention provides a protective element which comprises a heat-generating member and a low melting metal member on a substrate, the low melting metal member being caused to blow out by the heat generated by the heat-generating member, wherein a portion of the low melting metal member is formed on top of dummy electrodes so that a number of locations which are able to blow out is greater than the number of locations which are supposed to blow out.